1. Field of the Invention
The present invention relates to image processing devices, image processing methods, and storage media that store image processing programs, and particularly, to an image processing device that performs noise reduction processing on an image, an image processing method, and a computer-readable storage medium that stores an image processing program.
2. Description of Related Art
In the related art, gradation conversion processing is performed on image data so as to control the brightness and the contrast of the image scene. Examples of gradation conversion processing include so-called space-invariant gradation conversion processing in which gradation conversion processing is performed on the entire image on the basis of a single gradation conversion characteristic in the image scene, as in γ-correction, and so-called space-variant gradation conversion processing in which gradation conversion processing is performed by changing the gradation conversion characteristic for individual areas of the image scene so as to expand the dynamic range of the image scene.
However, with either kind of gradation conversion processing, although darker areas of the image scene can be made brighter, there is a problem in that noise in these areas is amplified at the same time. In light of this, Japanese Unexamined Patent Application, Publication No. 2007-312331 discloses a technology for reducing the noise amplified as the result of gradation conversion processing by increasing the magnitude of noise reduction if the gradation conversion characteristic used in the gradation conversion processing has a steep gradient.
On the other hand, performing noise reduction processing can create problems in that it impairs the image sharpness. Although the noise amplified as the result of gradation conversion processing can be reduced in the technology disclosed in Japanese Unexamined Patent Application, Publication No. 2007-312331, there is also a possibility that the image sharpness may be significantly impaired at the same time. Japanese Unexamined Patent Application, Publication No. Sho 61-157164 discloses a technology for performing noise reduction processing while preventing impairment of the image sharpness by detecting edges in a pre-noise-reduction image and then combining the pre-noise-reduction image with a post-noise-reduction image in accordance with the detected edges.